#include <fstream>
#include <iomanip>

int main()
{
   std::ifstream input("in.txt");
   std::ofstream output("out.txt");
   
   double s = 0;
   double t = 0;
   double dy = 0;
   double fx, fy;
   double lx, ly;
   double x, y;

   input >> fx >> fy;
   lx = fx;
   ly = fy;
   if (dy < -fy) {
      dy = -fy;
   }
   while (input >> x >> y)
   {
      s += (x - lx) * (y + ly) / 2;
      t += (x - lx);
      if (dy < -y)
      {
         dy = -y;
      }
      lx = x;
      ly = y;
   }
   s += (fx - lx) * (fy + ly) / 2;
   t += (fx - lx);

   s += t * dy;
   
   output << std::fixed;
   output << std::setprecision(8) << s;

   return 0;
}